Image forming apparatus

ABSTRACT

An image forming apparatus includes a first transport unit, a second transport unit, a cover, a flap, a drive unit for moving the flap, a first sensor for detecting opening and closing of the cover, a second sensor for detecting a position of a sheet, and a controller. The controller switches the drive unit to move the flap to the second position, upon determining it is necessary to move the flap to the second position, subsequent to determining that the cover is opened based on a detection result by the first sensor. The controller switches the drive unit to move the flap from the second position to the first position, upon determining, based on a detection result by the second sensor, that a leading edge of the sheet reaches a position downstream, in a sheet transport direction, of a swing end portion of the flap at the second position.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No.2018-243578 filed on Dec. 26, 2018, the content of which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

Aspects of the disclosure relate to an image forming apparatusconfigured to form an image on both sides of a sheet.

BACKGROUND

A known image forming apparatus capable of duplex image forming has amain transport path for transporting a sheet transported from an imageforming unit, a first discharge path for discharging the sheettransported from the main transport path to a discharge tray at an upperportion of a housing, and a return path branched from the main transportpath for transporting the sheet upward and switching back the sheet, andthen transporting the sheet downward back to the image forming unit.

The return path for inverting a sheet, which is provided separately fromthe first discharge path, allows the known image forming apparatus toform images on both sides of sheets at higher speed than an imageforming apparatus having a single path for discharging and inverting asheet.

SUMMARY

A known image forming apparatus has, in addition to a facedown dischargepath for discharging a sheet with an image-recorded surface facing downonto a discharge tray, a straight discharge path for dischargingstraight a sheet transported from an image forming unit to an exteriorof a housing. In this case, a curled sheet to be discharged straight mayerroneously enter the facedown discharge path.

Aspects of the disclosure provide an image forming apparatus configuredto discharge a sheet straight properly while preventing or reducingentry of a sheet into a first transport path leading to a discharge trayor into a second transport path branched from the first transport path.

According to one or more aspects of the disclosure, an image formingapparatus includes an image forming unit configured to form an image ona sheet, a housing, a first transport unit, a second transport unit, acover, a flap, a drive unit, a first sensor, a second sensor, and acontroller. The housing stores therein the image forming unit, includesa discharge tray at an upper portion thereof, and has a particular end.The first transport unit is configured to transport, along a firsttransport path, the sheet transported from the image forming unit to thedischarge tray. The second transport unit is configured to transport,along a second transport path, the sheet transported from the imageforming unit. The second transport path is branched from a junction withthe first transport path and extends upward at a position closer to theparticular end of the housing than the first transport path. The coveris disposed at the particular end of the housing pivotably relative tothe housing and configured to, when closed, at least partiallyconstitute the second transport unit and, when open, hold the sheettransported from the image forming unit to the exterior of the housing.The flap is disposed at the junction and swingable between a firstposition and a second position. The flap is configured to, when at thefirst position, block the second transport path and guide, along thefirst transport path, the sheet transported from the image forming unitand, when at the second position, block the first transport path andguide, along the second transport path, the sheet transported from theimage forming unit. The drive unit is configured to move the flapbetween the first position and the second position. The first sensor isconfigured to detect opening and closing of the cover. The second sensoris configured to detect a position of the sheet transported toward theexterior of the housing. The controller is configured to switch thedrive unit, based on results of detection by the first sensor and thesecond sensor, to move the flap. The controller is configured to switchthe drive unit to move the flap to the second position, upon determiningit is necessary to move the flap to the second position, subsequent todetermining that the cover is opened based on a detection result by thefirst sensor. The controller switches the drive unit to move the flapfrom the second position to the first position, upon determining, basedon a detection result by the second sensor, that a leading edge of thesheet reaches a position downstream, in a sheet transport direction, ofa swing end portion of the flap at the second position.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the disclosure are illustrated by way of example and not bylimitation in the accompanying figures in which like referencecharacters indicate similar elements.

FIG. 1 is a central cross-sectional view of an image forming apparatusaccording to an illustrative embodiment of the disclosure.

FIG. 2 is a cross-sectional side view of a transport unit of the imageforming apparatus, with a rear cover closed and a flap located at afirst position.

FIG. 3 is a cross-sectional side view of the transport unit, with therear cover closed and the flap located at a second position.

FIG. 4 is s cross-sectional side view of the transport unit, with therear cover open and the flap located at the second position.

FIG. 5 is a perspective view of the flap.

FIG. 6A is a side view of the flap switched into the first position byan electromagnetic solenoid.

FIG. 6B is a side view of the flap switched into the second position bythe electromagnetic solenoid.

FIG. 7 is a block diagram showing a first sensor, a second sensor, andthe electromagnetic solenoid which are connected to a controller.

FIG. 8 is a flowchart showing control by the controller to switch theelectromagnetic solenoid.

FIGS. 9A and 9B are schematic views of the flap moving from the secondposition to the first position after a leading edge of a sheet reaches afirst leading edge position.

FIGS. 10A and 10B are schematic views showing a swing end portion of theflap located at the first position pushes down a sheet to bend the sheetin a direction opposite to a direction in which the sheet is curled byimage forming.

FIG. 11A is a schematic side view showing that the flap at the firstposition is located upstream of an imaginary straight line.

FIG. 11B is a schematic side view showing that if the flap at the firstposition is located downstream of an imaginary straight line, a momentacts on the flap in a direction further pivoting toward the firstposition.

FIGS. 12A and 12B show a star wheel disposed at the swing end portion ofthe flap.

DETAILED DESCRIPTION

An illustrative embodiment of the disclosure will be described withreference to the accompanying drawings.

Overall Structure of Image Forming Apparatus

As shown in FIG. 1, an image forming apparatus according to anembodiment of the disclosure is an electrophotographic color laserprinter for forming an image on a sheet S, such as a paper sheet and atransparency, by overlapping developing agent images of differentcolors. The image forming apparatus may be a monochrome laser printerfor forming a monochrome developing agent image on a sheet.

In the following description, left and right sides of the page of FIG.1, a side facing out of the page, and a side facing into the page aredefined respectively as front, rear, left, and right sides of the imageforming apparatus 1. Upper and lower sides of the page of FIG. 1 aredefined respectively as upper and lower sides of the image formingapparatus 1.

The image forming apparatus 1 includes a housing 2, a feeder 3configured to feed a sheet S, an image forming unit 5 configured to forman image on the sheet S, and a transport unit 7 configured to transportthe sheet S transported from the image forming unit 5.

The housing 2 is a box having a substantially rectangular parallelepipedshape and stores therein the feeder 3, the image forming unit 5, and thetransport unit 7. The housing 2 includes a first opening 2A openrearward, and a rear cover 21 openable and closable relative to thefirst opening 2A. In other words, the rear cover 21 is disposed at arear end of the housing 2. The rear cover 21 is an example of a coverpivotable relative to a housing.

The rear cover 21 has a first pivot shaft or axis 21 a located at itslower end and extending in the left-right direction. The rear cover 21is pivotable about the first pivot axis 21 a to be opened and closedrelative to the housing 2. The first opening 2A is exposed when the rearcover 21 is open, and is covered when the rear cover 21 is closed. A topcover 23 covers an upper portion of the housing 2. The upper cover 23includes a discharge tray 23 a inclined downward from the front towardthe rear so as to be recessed. In other words, the housing 2 includes,at its upper portion, the discharge tray 23 a.

The feeder 3 includes a sheet cassette 31, a first feed roller 32, aseparation roller 33, a separation pad 33 a, a first transport rollerpair 34, and a registration roller pair 35. The housing 2 definestherein a main transport path P1 for a sheet S traveling from the sheetcassette 31 via the image forming unit 5.

The sheet cassette 31 supports a stack of sheets S. The first feedroller 32, the separation roller 33, and the separation pad 33 a feedone sheet S at a time from the sheet cassette 31 into the main transportpath P1. The transport rollers 34 and the registration rollers 35transport the sheet S fed into the main transport path P1 toward theimage forming unit 5.

The housing 2 includes a second opening 2B open frontward, and a frontcover 29 openable and closable relative to the second opening 2B. Thefront cover 29 has, at its lower end, a third pivot shaft or axis 29 aextending in the left-right direction. The front cover 29 is pivotableabout the third pivot axis 29 a to be opened and closed relative to thehousing 2. The second opening 2B is exposed when the front cover 29 isopen, and covered when the front cover 29 is closed.

The feeder 3 includes a second feed roller 38 and a second transportroller pair 39. In a state where the front cover 29 is open, the secondfeed roller 38 moves down and feeds one of the sheets S placed on thefront cover 29, and the second transport rollers 39 transport the sheetS into the main transport path P1. The registration rollers 35 transportthe sheet S fed into the main transport path P1 toward the image formingunit 5.

The image forming unit 5 is disposed above the feeder 3 and includesfour drum units 51 arranged in tandem in the front-rear direction. Eachdrum unit 51 corresponds to one of colors: black, yellow, magenta, andcyan. Each drum unit 51 includes a photosensitive drum 51 a, a charger51 b, and a developing roller 51 c.

The image forming unit 5 includes a scanner 52 and a fixer 60. Thescanner 52 is disposed at an upper portion in the housing 2 andirradiates each photosensitive drum 51 based on image data by scanningat high speed a laser beam over its surface, via a polygon mirror,lenses, and reflection mirrors. The fixer 60 is disposed downstream ofthe most downstream one of the photosensitive drums 51 a in a sheettransport direction.

A transfer belt 40 is disposed below the image forming unit 5 to definethe main transport path P1 therebetween. The transfer belt 40 isstretched between a drive roller 41 a and a driven roller 41 b which isdisposed further to the rear of the housing 2 than the drive roller 41a. A transfer roller 42 is disposed opposite to each photosensitive drum51 a with the transfer belt 40 therebetween.

In the image forming unit 5, the scanner 52 selectively irradiates aphotosensitive drum 51 a which is uniformly charged by a correspondingcharger 51 b. This irradiation selectively removes electric charges fromthe photosensitive drum 51 a, thereby forming an electrostatic latentimage on a surface of the photosensitive drum 51 a.

A developing bias is applied to each developing roller 51 c. When anelectrostatic latent image formed on the photosensitive drum faces acorresponding developing roller 51 c, the developing roller 51 csupplies toner to the electrostatic latent image because of the electricpotential difference. Consequently, a toner image is formed on thephotosensitive drum 51 a.

A sheet S transported by the transfer belt 40 toward the image formingunit 5 sequentially passes between the transfer belt 40 and thephotosensitive drums 51 a. When each photosensitive drum 51 a faces thesheet, a toner image on a surface thereof is transferred onto the sheetS because of a transfer bias applied to a corresponding transfer roller42.

The sheet S with the transferred toner images is transported to thefixer 60. The fixer 60 includes a heat roller 61 for heating the sheetS, and a pressure roller 62 disposed opposite to the heat roller 61. Theheat roller 61 and the pressure roller 62 constitute a fixing rollerpair. When the sheet S transported to the fixer 60 passes between theheat roller 61 and the pressure roller 62 which are in press-contactwith each other, the toner images are thermally fixed to the sheet S.

The transport unit 7 transports the sheet S with the thermally fixedtoner images downstream away from the image forming unit 5. Thetransport unit 7 includes a first transport unit 71 and a secondtransport unit 72.

The first transport unit 71 has a function of discharging a sheet Stransported from the image forming unit 5 onto the discharge tray 23 a.The second transport unit 72 has a function of returning a sheet Stransported from the image forming unit 5 back to the image forming unit5.

Instead of returning a sheet S to the image forming unit 5, the secondtransport unit 72 may have a function of transporting a sheet Stransported from the image forming unit 5 toward an optional devicedisposed above the image forming apparatus 1. For example, an optionaldevice may be a mail box including multiple sheet trays.

In a duplex mode where the image forming apparatus 1 forms images onboth sides of a sheet S, the second transport unit 72 inverts andtransports a sheet S having an image formed on one side by the imageforming unit 5 back to the image forming unit 5. After the image formingunit 5 forms an image on the other side of the sheet S, the firsttransport unit 71 discharges the sheet S onto the discharge tray 23 a.As compared with a case where the first transport unit 71 alonedischarges and returns sheets S, the second transport unit 72, which isdedicated to return sheets S to the image forming unit 5, allows for theimage forming apparatus 1 to transport more sheets per unit time in theduplex mode.

Transport Unit

The transport unit 7 will now be described. As shown in FIGS. 1-4, thetransport unit 7 includes the first transport unit 71, the secondtransport unit 72, a flap 73, a first sensor 91, and a second sensor 92.

The first transport unit 71 includes a post-fixing roller pair 710, anintermediate discharge roller pair 711, and a discharge roller pair 712which are disposed in this order on a downstream side of the fixer 60 inthe sheet transport direction to transport a sheet S. The dischargeroller pair 712 discharges the sheet S onto the discharge tray 23 a. Thepost-fixing roller pair 710 is an example of a second transport rollerpair which is disposed downstream of an image forming unit in a sheettransport direction to transport a sheet. The post-fixing roller pair710 is disposed at an upper rear portion of the heat roller 61 and thepressure roller 62.

The first transport unit 71 defines a main transport path P21 fortransporting a sheet S from the fixer 60 to the post-fixing roller pair710, and a first discharge path P22 for discharging the sheet S guidedfrom the post-fixing roller pair 710 onto the discharge tray 23 a. Afirst transport path P2 of the first transport unit 71 includes the maintransport path P21 and the first discharge path P22. A sheet Stransported from the image forming unit is transported onto thedischarge tray 23 a along the first transport path P2. The maintransport path P21 and the first discharge path P22 are each an exampleof a first transport path.

The main transport path P21 extends obliquely rearward and upward fromthe fixer 60 to the post-fixing roller pair 710. The first dischargepath P22 extends upward from a downstream end of the main transport pathP21, and then obliquely frontward and upward toward the discharge tray23 a. The intermediate discharge roller pair 711 is disposed in themiddle of the first discharge path P22. The discharge roller pair 712 isdisposed at a downstream end of the first discharge path P22.

In discharging a sheet S transported from the image forming unit 5 ontothe discharge tray 23 a in the image forming apparatus 1, thepost-fixing roller pair 710 transports the sheet S having a toner imagethermally fixed by the fixer 60 from the main transport path P21 intothe first discharge path P22. Then the intermediate discharge rollerpair 711 transports the sheet S along the first discharge path P22, andthe discharge roller pair 712 discharges the sheet S onto the dischargetray 23 a.

The second transport unit 72 defines a return path P3 which is branchedfrom the first transport path P2 and along which a sheet S transportedfrom the image forming unit 5 is transported back toward the imageforming unit 5. The return path P3 is an example of a second transportpath. The first transport path P2 and the return path P3 branch at ajunction Ps. The return path P3 includes a first path P31 fortransporting a sheet S transported from the image forming unit 5, and asecond path P32 for returning a switched back sheet S toward the imageforming unit 5. The first path P31 branches at the junction Ps from thefirst transport path P2 and extends upward at a position further to therear of the housing 2 than the first transport path P2. The second pathP32 branches from the first path P31 and extends downward. The firstpath P31 extends to an upper end of the housing 2. The second path P32branches from the first path P31, extends downward, and then extendsfrontward below the sheet cassette 31 to reach the first transportroller pair 34.

The second transport unit 72 includes a first switchback roller pair 721and a second switchback roller pair 722 which define the first path P31of the return path P3. The first switchback roller pair 721 is anexample of a first transport roller pair disposed at the secondtransport unit to transport a sheet. The second switchback roller pair722 is disposed above the first switchback roller pair 721. The firstswitchback roller pair 721 and the second switchback roller pair 722 aresupported by the housing 2. These switchback roller pairs 721 and 722remain supported by the housing 2 in a state where the rear cover 21 isopen.

The first switchback roller pair 721 and the second switchback rollerpair 722 are configured to rotate in forward and reverse directions. Thefirst and second switchback roller pairs 721 and 722 transport a sheet Salong the first path P31 to an exterior of the housing 2 when rotatingin the forward direction, and toward an interior of the housing 2 whenrotating in the reverse direction.

The flap 73 is disposed at the junction Ps between the first transportpath P2 and the return path P3. The flap 73 has, at its end, a firstswing shaft or axis 73 a parallel to the first pivot axis 21 a and has,at its other end, a swing end portion 73 b. The swing end portion 73 bof the flap 73 is supported by the housing 2 swingably about the firstswing axis 73 a. The first swing axis 73 a is located above the junctionPs and the swing end portion 73 b.

The flap 73 is configured to swing about the first swing axis 73 abetween a first position (shown in FIG. 2) and a second position (shownin FIG. 3). The flap 73, when located at the first position, guides asheet S transported from the image forming unit 5 into the firstdischarge path P22 and blocks the first path P31 of the return path P3and, when located at the second position, guides the sheet S into thefirst path P31 and blocks the first discharge path P22. The swing endportion 73 b is located at an upper position when the flap 73 is at thesecond position than when the flap 73 is at the first position.

The heat roller 61 and the pressure roller 62 of the fixer 60 define afirst nip N1 therebetween. The post-fixing roller pair 710 definestherebetween a second nip N2. The second nip N2 is located downstream ofand above the first nip N1. The flap 73 is disposed downstream of thepost-fixing roller pair 710.

The swing end portion 73 b of the flap 73 having moved to the secondposition is located on an upper side of a tangent Nt to the second nipN2 between the post-fixing roller pair 710 (refer to the flap 73 shownby a solid line in FIG. 4). In other words, the swing end portion 73 bof the flap 73 at the second position is located opposite to the openfirst cover 21 relative to the tangent Nt. The swing end portion 73 b ofthe flap 73 having moved to the first position is located on a lowerside of the tangent Nt to the second nip N2 between the post-fixingroller pair 710 (refer to the flap 73 shown by a two-dot chain line inFIG. 4).

The first sensor 91 is disposed at an upper end of the opening 2A of thehousing 2 to detect opening and closing of the rear cover 21. The secondsensor 92 is disposed downstream of the post-fixing roller pair 710 inthe sheet transport direction to detect the position of a sheet S beingtransported. The second sensor 92 is configured to detect a leading edgeof a sheet transported by the post-fixing roller pair 710.

The housing 2 has a first guide surface 271, a second guide surface 272,and a third guide surface 273. The first guide surface 271 partiallydefines the first transport path P2 in the first transport unit 71. Thesecond guide surface 272 is located further to the rear (i.e., closer tothe rear end) of the housing 2 than the first guide surface 271 andpartially defines the first transport path P2. The third guide surface273 is located further to the rear (i.e., closer to the rear end) of thehousing 2 than the second guide surface 272 and partially defines thereturn path P3. The rear cover 21 has a fourth guide surface 211 locatedfurther to the rear (i.e., closer to the rear end) of the housing 2 thanthe third guide surface 273 to partially define the return path P3.

The rear cover 21, when closed, defines at least a portion of the returnpath P3 and, when open, holds a sheet S transported from the imageforming unit 5 to an exterior of the housing 2. When the rear cover 21is open, the post-fixing roller pair 710 transports the sheettransported from the image forming unit 5 rearward to a position outsideof the housing 2. In this case, the sheet S is transported to the rearcover 21 along a second discharge path P4, thereby being dischargedstraight.

Drive Unit of Flap

As shown in FIGS. 5 and 6, the flap 73 includes, at its right end, anoperation lever 73 c extending rearward. The flap 73 is configured toswing to the first position by its own weight in a free state where noforce is applied to the operation lever 73 c in a pivoting direction.

The flap 73 includes a first flap member 731 disposed at its centralportion in a width direction orthogonal to the sheet transportdirection, and second flap members 732 disposed closer to its widthwiseends than the first flap member 731. A plurality of second flap members732 are disposed on the left and right sides of the first flap member731. When viewed in an axial direction of the first swing axis 73 a, thelength of each second flap member 732 from the first swing axis 73 a toa swing end 732 b is less than the length of the first flap member 731from the first swing axis 73 a to a swing end 731 b. The swing end 732 bof each second flap member 732 is located above the swing end 731 b ofthe first flap member 731.

This may prevent or reduce interference of the flap 73 with other partsof the image forming apparatus 1 more than when the swing end 732 b ofeach second flap member 732 is level with the swing end 731 b of thefirst flap member 731. The flap 78 has a less moment of inertia whenmoving between the first position and the second position and thusreadily moves therebetween.

Further, sheets pass through a central portion of the flap 73 in thewidth direction, regardless of the sheet size and type, and thus arereliably pushed by the flap 73.

The image forming apparatus 1 includes an electromagnetic solenoid 74for driving the flap 73, and a drive lever 75 connected to theelectromagnetic solenoid 74 so as to contact and swing the flap 73. Theelectromagnetic solenoid 74 is an example of a drive unit configure tomove the flap 73 between the first position and the second position. Theelectromagnetic solenoid 74 and the drive lever 75 are supported by thehousing 2.

The electromagnetic solenoid 74 includes an extendable plunger 74 aconnected to the drive lever 75. The drive lever 75 includes, at its oneend, a second swing shaft or axis 75 a and, at its other end, a contactportion 75 b contactable with the operation lever 73 a. When the drivelever 75 swings about the second swing axis 75 a, the contact portion 75b is movable between a spaced position at which the contact portion 75 bis spaced from the operation lever 73 c, and a contact position at whichthe contact portion 75 b contacts the operation lever 73 c and pushesthe operation lever 73 c in the pivoting direction.

The plunger 74 a extends when the electromagnetic solenoid 74 isunexcited, and retracts when the electromagnetic solenoid 74 is excited.The drive lever 75 moves to the spaced position when the plunger 74 aextends, and moves to the contact position when the plunger 74 aretracts.

As shown in FIG. 6A, when the electromagnetic solenoid 74 is unexcited,the drive lever 75 moves to the spaced position such that the contactportion 75 b moves away from the operation lever 73 c. This makes theflap 73 into a free state, thereby allowing the flap 73 to swing to thefirst position by its own weight. On the other hand, as shown in FIG.6B, when the electromagnetic solenoid 74 is excited, the drive lever 75moves to the contact position 75 b such that the contact portion 75 bpushes the operation lever 73 c in the pivoting direction. Thus, theflap 73 swings from the first position to the second position. Theelectromagnetic solenoid 74 is configured to, when unexcited, switch theflap 73 into the first position and, when excited, switch the flap 73into the second position.

As shown in FIG. 7, the image forming apparatus 1 includes a controller9 which switches the electromagnetic solenoid 74 to move the flap 73,based on the results of detection by the first sensor 91 and the secondsensor 92. The first sensor 91, the second sensor 92, and theelectromagnetic solenoid 74 are connected to the controller 9.

In order for the image forming unit 5 to guide a sheet S into the firstdischarge path P22, the controller 9 switches the electromagneticsolenoid 74 into an unexcited state to cause the flap 73 to swing to thefirst position and block the first path P31 of the return path P3. Inorder for the image forming unit 5 to guide a sheet S into the firstpath P31 of the return path P3, the controller 9 switches theelectromagnetic solenoid 74 into an excited state to cause the flap 73to swing to the second position and block the first discharge path P22.

In the image forming apparatus 1, a time period for transporting a sheetS along the first transport path P2 with the flap 73 located at thefirst position is likely to be longer than a time period fortransporting a sheet S along the return path P3 with the flap 73 locatedat the second position. Thus, the flap 73 is configured to be located atthe first position when the electromagnetic solenoid 74 is unexcited.This may reduce a time period during which the electromagnetic solenoid74 is excited, resulting in energy saving.

Controller for Switching Electromagnetic Solenoid

When sheets are discharged to an exterior of the housing 2 with the rearcover 21 open, the controller 9 executes switching of theelectromagnetic solenoid 74, to thereby move the flap 73.

When the first sensor 91 detects opening of the rear cover 21, thecontroller 9 switches the electromagnetic solenoid 74 to move the flap73 to the second position. When the second sensor 92 detects theposition of a sheet S and the controller 91 determines that a leadingedge of the sheet S reaches a position downstream of the swing endportion 73 b of the flap 73, the controller 9 switches theelectromagnetic solenoid 74 to move the flap 73 from the second positionto the first position.

Referring now to FIG. 8, control by the controller 9 to switch theelectromagnetic solenoid 94 will be described in detail. Theelectromagnetic solenoid 94 is initially switched in an unexcited state.

First, the controller 9 determines whether the first sensor 91 detectsopening of the rear cover 21 (step S01). Upon determining that the firstsensor 91 does not detect opening of the rear cover 21 in step S01 (stepS01: N), the controller 9 executes step S01 again.

In contrast, upon determining that the first sensor 91 detects openingof the rear cover 21 in step S01 (step S01: Y), the controller 9determines whether the flap 73 is located at the second position (stepS02). The position of the flap 73 is determined based on a status of theelectromagnetic solenoid 74. Upon determining that the flap 73 islocated at the second position in step S02 (step S02: Y), the controller9 determines that it is not necessary to move the flap 73. Subsequently,the controller 9 determines whether there is a job for the image formingunit 5 (step S03).

Upon determining that the flap 73 is not located at the second positionin step S02 (step S02: N), the controller 9 determines that it isnecessary to move the flap 73 and switches the electromagnetic solenoid74 from unexcited to excited to move the flap 73 to the second position(step S04).

In short, when the controller 9 determines that the rear cover 21 isopened based on the detection result by the first sensor 91 and that itis necessary to move the flap 73, the controller 9 switches theelectromagnetic solenoid 74 to move the flap 73 to the second position.

After moving the flap 73 to the second position, the controller 9determines whether there is a job for the image forming unit 5. Upondetermining that there is no job for the image forming unit 5 in stepS03 (step S03: N), the controller 9 executes step S03 again. Incontrast, upon determining that there is a job for the image formingunit 5 in step S03 (step S03: Y), the controller 9 determines whetherthe second sensor 92 detects a sheet S (step S05). Upon determining thatthe second sensor 92 does not detect a sheet S in step S05 (step S05:N), the controller 9 executes step S5 again.

When the image forming apparatus 1 executes a job, the image formingunit 1 forms an image on a sheet S and the sheet S with the image formedthereon is transported downstream from the image forming unit 5. Whenthe post-fixing roller pair 710 transports the sheet S downstream, thesecond sensor 92 detects a leading edge of the sheet S.

Upon determining that the second sensor 92 detects the sheet S in stepS05 (step S05: Y), the controller 9 determines whether a leading edge ofthe sheet S reaches a first leading edge position. Herein, the firstleading edge position refers to a position where a leading edge of asheet S reaches the swing end portion 73 b of the flap 73 hypotheticallylocated at the first position. In other words, the first leading edgeposition coincides with the position of the swing end portion 73 b ofthe flap 73 located at the first position.

For example, the controller 9 may calculate, using the transport speedof a sheet S and a distance from the second sensor 92 to the firstleading edge position, a time period taken for a leading edge of a sheetS to reach the first leading edge position after the detection of theleading edge by the second sensor 92. In this case, the controller 9determines that the leading edge of the sheet S reaches the firstleading edge position upon elapse of the calculated time period afterthe detection of the leading edge by the second sensor 92.

In other words, the controller 9 may determine a lapsed time since thesecond sensor 92 detects the leading edge of the sheet S. Then, thecontroller 9 may determine whether the lapsed time reaches a particulartime. If the lapsed time reaches the particular time, the controller 9determines that the leading edge of the sheet S reaches the firstleading edge position. The particular time is determined such that theleading edge of the sheet S reaches the first leading edge position, ata timing when the particular time elapses since the second sensor 92detects the leading edge of the sheet S, where the swing end portion 73b of the flap 73 would locate when the flap 73 is at the first position.

The particular time X is expressed by the following formula:

X=Z/Y, where Y is the transport speed of a sheet S, and Z is a distancefrom the second sensor 92 to the first leading edge position.

Upon determining that the leading edge of the sheet S does not reach thefirst leading edge position in step S06 (step S06: N), the controller 9executes step S06 again. In contrast, upon determining that the leadingedge of the sheet S reaches the first leading edge position in step S06(step S06: Y), the controller 9 determines that the leading edge of thesheet S reaches a position downstream of the swing end portion 73 b ofthe flap 73 and switches the electromagnetic solenoid 74 from excited tounexcited to move the flap 73 from the second position to the firstposition (step S07).

In short, when the controller 9 determines that the leading edge of thesheet S reaches the position downstream of the swing end portion 73 b ofthe flap 73 at the second position, based on the detection result by thesecond sensor 92, the controller 9 switches the electromagnetic solenoid74 to move the flap 73 from the second position to the first position.

When sheets are discharged straight to an exterior of the housing 2, thecontroller 9 controls switching of the electromagnetic solenoid 74 so asto keep the flap 73 at the second position until the leading edge of thesheet S reaches a position downstream of the swing end portion 73 b ofthe flap 73. The flap 73 at the second position blocks the firsttransport path P2 to prevent entry of the leading edge of the sheet Sinto the first transport path P2. After the leading edge of the sheet Sreaches the position downstream of the swing end portion 73 b of theflap 73, the flap 73 moves from the second position to the firstposition to block the return path P3, thereby preventing entry of thesheet S into the return path P3. This may enable proper transport of thesheet S to an exterior of the housing 2.

In this case, if the swing end portion 73 b of the flap 73 overtakes aleading edge of a sheet S and moves into a position downstream of theleading edge, the sheet S may enter the first transport path P2.

In this embodiment, however, as shown in FIG. 9A, the flap 73 is causedto move from the second position to the first position after a leadingedge of a sheet S reaches the first leading edge position. Thus, asshown in FIG. 9B, the swing end portion 73 b of the flap 73 moving tothe first position is unlikely to overtake the leading edge of the sheetS. This may further prevent entry of the sheet S into the firsttransport path P2 and the return path P3, enabling proper transport ofthe sheet S.

Particularly, as shown in FIG. 9A, by moving the flap 73 from the secondposition to the first position after a leading edge of a sheet S reachesthe position of an upstream side surface 73 d of the swing end portion73 b of the flap 73 hypothetically located at the first position, theswing end portion 73 b of the flap 73 is unlikely to overtake theleading edge of the sheet S. This may prevent entry of the sheet S intothe first transport path P2 and the return path P3.

When the rear cover 21, which partially constitutes the second transportunit 72, is opened, the return path P3 is exposed and the firstswitchback roller pair 721 remains supported by the housing 2. Unlike inthis embodiment, if the flap 73 is located at the second position when asheet S is transported downstream by the post-fixing roller pair 710with the rear cover open 21, the sheet S may enter the first path P31 ofthe return path P3 and be transported by the first switchback rollerpair 721.

In this embodiment, however, when the leading edge of the sheet Sreaches to a position downstream of the swing end portion 73 b of theflap 73, the flap 73 is caused to move from the second position to thefirst position to block the return path P3, thereby preventing entry ofthe sheet S into the return path P3. This may enable proper transport ofthe sheet S. When the rear cover 21 is opened, the first switchbackroller pair 721 remains at the housing 2, instead of moving togetherwith the rear cover 21. This may simplify in structure the firstswitchback roller pair disposed at the housing 2.

After determining that a leading edge of a sheet S has reached aposition downstream of the swing end portion 73 b of the flap 3, thecontroller 9 causes the flap 73 to move from the second position to thefirst position. As shown in FIG. 4, the flap 73 moving from the secondposition to the first position crosses the tangent Nt to the nip N2between the post-fixing roller pair 710.

By crossing the nip tangent Nt, the swing end portion 73 b of the flap73 is allowed to move in contact with the sheet S. Thus, the flap 73moves from the second position to the first position, the swing endportion 73 b pushes down the sheet S. This may further prevent entry ofthe sheet S into the return path P3 and enable proper transport of thesheet S.

If the image forming apparatus 1 is not provided with the post-fixingroller pair 710, entry of a sheet S into the return path P3 may beprevented by the flap 73 which is configured to, when moving from thesecond position to the first position, cross a tangent to the first nipN1 between the heat roller 61 and the pressure roller 62.

As shown in FIG. 10A, in the image forming apparatus 1, the swing endportion 73 b of the flap 73 at the second position is located on anupper side of a nip straight line Ns passing through the first nip N1between the heat roller 61 and the pressure roller 62 and the second nipN2 between the post-fixing roller pair 710. As shown in FIG. 10B, theswing end portion 73 b of the flap 73 at the first position is locatedon a lower side of the nip straight line Ns.

As shown in FIG. 10A, when a sheet S having an image formed thereon bythe image forming unit 5 is heated by the fixer 60 for image fixing, thesheet S tends to be curled such that a concave sheet surface faces up.As shown in FIG. 10B, the flap 73 is configured such that, when at thefirst position, the swing end portion 73 b is located on a lower side ofthe nip straight line Ns. The swing end portion 73 b of the flap 73 atthe first position pushes down the sheet S at a position downstream ofthe post-fixing roller pair 710. The swing end portion 73 b bends thesheet S such that a concave sheet surface faces down. In other words,the swing end portion 73 b bends the sheet S in a direction opposite toa direction in which the sheet S is curled, thereby correcting curlingof the sheet S.

As shown in FIG. 11A, the flap 73 at the first position is locatedupstream, in the first path P31 of the return path P3, of an imaginarystraight line Vs passing through the first swing axis 73 a andperpendicular to the sheet S being transported.

As shown in FIG. 11B, when the swing end portion 73 b of the flap 73contacts the sheet S transported along the first path P31 of the returnpath P3, the flap 73 receives an upward force F from the sheet S. Inthis case, if the swing end portion 73 b of the flap 73 is locateddownstream of the imaginary straight line Vs in the sheet transportdirection, a moment Mf acts on the flap 73 in a direction pivoting aboutthe first swing axis 73 a beyond the first position. Nevertheless, theflap 73 is unable to pivot any further and may be broken.

In contrast, as shown in FIG. 11A, the swing end portion 73 b of theflap 73 is located upstream of the imaginary straight line Vs in thesheet transport direction, a moment Mb acts on the flap 73, due to aforce from the sheet S, in a direction pivoting about the first swingaxis 73 a toward the second position. The flap 73 is pivotable from thefirst position to the second position without being broken.

As shown in FIG. 12A, a star wheel 78 is disposed at the swing endportion 73 b of the flap 73 rotatably about an axis parallel to thefirst swing shaft or axis 73 a. When the swing end portion 73 b of theflap 73 approaches a sheet S, the star wheel 78 contacts the sheet S.The star wheel 78 is an example of a roller. As shown in FIG. 8B, thestar wheel 78 includes, at its outer circumference, a plurality ofcontact teeth 78 a contactable with the sheet S.

The star wheel 78 rotatably disposed at the swing end portion 73 brotates when the sheet S contacts the swing end portion 73 b. This mayreduce the transport resistance to the sheet S and the sliding noisegenerated between the flap 73 and the transported sheet S, enablingproper transport of the sheet S.

Especially, when the star wheel 78 is used as a roller, the contactteeth 78 a intermittently contact the sheet S, thereby further reducingthe contact area with the sheet S and the transport resistance. When theswing end portion 73 b of the flap 73 contacts an image-formed surfaceof the sheet S, the star wheel 78 has a reduced contact area with thesheet S and thus is less likely to affect the image-formed surface.

Although, in this embodiment, the second sensor 92 for detecting theposition of a transported sheet S is disposed downstream of thepost-fixing roller pair 710, the second sensor may be disposedelsewhere. For example, the second sensor 92 may be disposed downstreamof the registration roller pair 35.

When the second sensor 92 is disposed downstream of the registrationroller pair 35, the controller 9 may calculate, using the transportspeed of a sheet S, the length of a sheet S in the transport direction,and a distance from the second sensor to the first leading edgeposition, a time period taken for a leading edge of the sheet S to reachthe first leading edge position after the detection of a trailing edgeof the sheet S by the second sensor 92. In this case, the controller 9determines that the leading edge of the sheet S reaches the firstleading edge position upon elapse of the calculated time period afterthe detection of the trailing edge by the second sensor 92. In thiscase, the controller 9 may calculate the length of the sheet S in thetransport direction, using a time period from the detection by thesecond sensor of the leading edge of the sheet S until the detection ofthe trailing edge of the sheet S, and the transport speed of the sheetS.

While the disclosure has been described in detail with reference toparticular examples, various changes, arrangements and modifications maybe applied therein without departing from the spirit and scope of thedisclosure.

What is claimed is:
 1. An image forming apparatus comprising: an imageforming unit configured to form an image on a sheet; a housing storingtherein the image forming unit and including a discharge tray at anupper portion thereof, the housing having a particular end; a firsttransport unit configured to transport, along a first transport path,the sheet transported from the image forming unit to the discharge tray;a second transport unit configured to transport, along a secondtransport path, the sheet transported from the image forming unit, thesecond transport path being branched from a junction with the firsttransport path and extending upward at a position closer to theparticular end of the housing than the first transport path; a coverdisposed at the particular end of the housing pivotably relative to thehousing and configured to, when closed, at least partially constitutethe second transport unit and, when open, hold the sheet transportedfrom the image forming unit to an exterior of the housing; a flapdisposed at the junction and swingable between a first position and asecond position, the flap being configured to: when at the firstposition, block the second transport path and guide, along the firsttransport path, the sheet transported from the image forming unit, andwhen at the second position, block the first transport path and guide,along the second transport path, the sheet transported from the imageforming unit; a drive unit configured to move the flap between the firstposition and the second position; a first sensor configured to detectopening and closing of the cover; a second sensor configured to detect aposition of the sheet transported toward the exterior of the housing;and a controller configured to switch the drive unit, based on resultsof detection by the first sensor and the second sensor, to move theflap, wherein the controller is configured to: switch the drive unit tomove the flap to the second position, upon determining it is necessaryto move the flap to the second position, subsequent to determining thatthe cover is opened based on a detection result by the first sensor, andswitch the drive unit to move the flap from the second position to thefirst position, upon determining, based on a detection result by thesecond sensor, that a leading edge of the sheet reaches a positiondownstream, in a sheet transport direction, of a swing end portion ofthe flap at the second position.
 2. The image forming apparatusaccording to claim 1, wherein the second transport unit includes a firsttransport roller pair configured to transport the sheet, the firsttransport roller pair being supported by the housing when the cover isopen.
 3. The image forming apparatus according to claim 1, wherein thefirst transport unit includes a second transport roller pair disposeddownstream of the image forming unit in the sheet transport directionand configured to transport the sheet, wherein the flap is configured tocross a tangent to a nip between the second roller pair when the flap ismoved from the second position to the first position upon determinationby the controller that the leading edge of the sheet reaches theposition downstream of the swing end portion of the flap at the secondposition.
 4. The image forming apparatus according to claim 1, whereinthe controller is configured to switch the drive unit to move the flapfrom the second position to the first position upon determining, basedon the detection result by the second sensor, that the leading edge ofthe sheet reaches a first leading edge position which coincides with aposition of the swing end portion of the flap located at the firstposition.
 5. The image forming apparatus according to claim 1, whereinthe flap at the first position is located upstream, in the sheettransport direction along the second transport path, of an imaginarystraight line passing through a swing axis of the flap and perpendicularto the sheet being transported.
 6. The image forming apparatus accordingto claim 1, wherein the image forming unit is configured to place on thesheet a monochrome developing agent image or developing agent images ofdifferent colors, wherein the image forming apparatus further comprises:a fixing roller pair defining a first nip therebetween; and a secondtransport roller pair disposed downstream of the fixing roller pair inthe sheet transport direction and defining therebetween a second nipwhich is located upstream of the first nip, and wherein the flap isdisposed downstream of the second transport roller pair and the swingend portion of the flap at the first position is located on a lower sideof a straight line passing through the first nip and the second nip. 7.The image forming apparatus according to claim 1, wherein the flapincludes a first flap member disposed at a central portion thereof in awidth direction orthogonal to the sheet transport direction, and asecond flap member disposed closer to a widthwise end thereof than thefirst flap member, and a length from a swing axis of the flap to a swingend of the second flap member is less than a length from the swing axisof the flap to a swing end of the first flap member.
 8. The imageforming apparatus according to claim 1, wherein the flap includes arotatable roller disposed at the swing end portion thereof.
 9. An imageforming apparatus comprising: an image forming unit configured to forman image on a sheet; a housing storing therein the image forming unitand including a discharge tray at an upper portion thereof, the housinghaving a particular end; a first transport unit configured to transport,along a first transport path, the sheet transported from the imageforming unit to the discharge tray; a second transport unit configuredto transport, along a second transport path, the sheet transported fromthe image forming unit, the second transport path being branched from ajunction with the first transport path and extending upward at aposition closer to the particular end of the housing than the firsttransport path; a cover disposed at the particular end of the housingand movable between an open position and a closed position; a flapdisposed at the junction and swingable between a first position and asecond position, the flap being configured to: when at the firstposition, block the second transport path and guide, along the firsttransport path, the sheet transported from the image forming unit, andwhen at the second position, block the first transport path and guide,along the second transport path, the sheet transported from the imageforming unit; a drive unit configured to move the flap between the firstposition and the second position; a first sensor configured to detectwhether the cover is open; a second sensor configured to detect aleading edge of the sheet; and a controller configured to: determinewhether the flap is at the first position or the second position basedon status of the drive unit; determine a lapsed time since the secondsensor detects the leading edge of the sheet; upon determining that theflap is at the first position and that the cover is open, control thedrive unit to move the flap to the second position; and upon determiningthat the flap is at the second position and the lapsed time is aparticular time, control the drive unit to move the flap to the firstposition, wherein the particular time is determined such that theleading edge of the sheet reaches a position, at a timing when theparticular time elapses since the second sensor detects the leading edgeof the sheet, where a swing end portion of the flap would locate whenthe flap is at the first position.